In recent years, small-sized electronic devices represented by mobile terminals and the like have been widely spread and further down-sizing, lighter weight and longer life are strongly demanded. To a market demand like this, secondary batteries capable of obtaining, in particular, a smaller size, a lighter weight and a higher energy density have been developed. The secondary batteries are studied to be applied also to, without limiting to small-sized electronic devices, large-sized electronic devices represented by automobiles and power-storage systems represented by houses or the like.
Among these, a lithium ion secondary battery is highly expected because a smaller size and higher capacity are easy to be obtained and the energy density higher than that of a lead battery or a nickel-cadmium battery can be obtained.
The lithium ion secondary battery comprises a positive electrode, a negative electrode, a separator, and an electrolytic solution. The negative electrode comprises a negative electrode active material related to a charge/discharge reaction.
As the negative electrode active material, while a carbon material is widely used, a further improvement in a battery capacity is demanded from recent market demand. As a factor for improving the battery capacity, it has been studied to use silicon as the negative electrode active material. This is because a great improvement of the battery capacity may be expected since silicon has a theoretical capacity (4199 mAh/g) no smaller than 10 times a theoretical capacity of graphite (372 mAh/g). A development of a silicon material as the negative electrode active material comprises studies on not only a silicon simple substance but also on compounds represented by alloys, oxides or the like. Further, shapes of the active material have been studied, from a coating type, which is standard in the carbon material, to an integrated type directly deposited on a current collector.
However, when, as the negative electrode active material, the silicon is used as a main raw material, since particles of the negative electrode active material expand and contract during charge/discharge, mainly the neighborhood of a superficial layer of the particles of negative electrode active material tends to crack. Further, an ionic substance is generated inside of the active material, and the particles of negative electrode active material tends to be broken. When a superficial layer of the particles of negative electrode active material is broken, a new surface is generated thereby, and a reaction area of the particles of the negative electrode active material increases. At this time, since a decomposition reaction of an electrolytic solution occurs on the new surface and a film that is a decomposition product of the electrolytic solution is formed on the new surface, the electrolytic solution is consumed. Therefore, cycle characteristics of the battery tends to be degraded.
Until now, in order to improve an initial efficiency and the cycle characteristics of a battery, negative electrode materials for lithium ion secondary batteries comprising the silicon material as a main material and electrode configurations have been variously studied.
Specifically, in order to obtain excellent cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited by using a vapor phase method (see, for example, Patent Document 1 below). Further, in order to obtain high battery capacity and safety, a carbon material (an electron conducting material) is provided on a superficial layer of particles of silicon oxide (see, for example, Patent Document 2). Further, in order to improve the cycle characteristics and to obtain high input/output characteristics, an active material containing silicon and oxygen is prepared and an active material layer comprising a high oxygen ratio in the neighborhood of a current collector is formed (see, for example, Patent Document 3). Still further, in order to improve the cycle characteristics, oxygen is contained in a silicon active material such that an average oxygen content is 40 atomic percent or lower, and an oxygen content is high in a place close to a current collector (see, for example, Patent Document 4).
Further, in order to improve an initial charge/discharge efficiency, a nano composite containing a Si phase, SiO2 and a MyO metal oxide is used (see, for example, Patent Document 5). Still further, in order to improve the initial charge/discharge efficiency, pre-doping in which a Lithium-containing material is added to a negative electrode, and Lithium is decomposed in a place where a negative electrode potential is high and is returned to a positive electrode is performed (see, for example, Patent Document 6).
Still further, in order to improve the cycle characteristics, SiOx (0.8≤x≤1.5, a particle size range=1 μm to 50 μm) and a carbon material are mixed and baked at a high temperature (see, for example, Patent Document 7). Further, in order to improve the cycle characteristics, a mole ratio of oxygen to silicon in a negative electrode active material is set to from 0.1 to 1.2, and, in the neighborhood of an interface of the active material and a current collector, an active material is controlled in the range where a difference of a maximum value and a minimum value of the mole ratios of oxygen amounts to silicon amounts is 0.4 or smaller (see, for example, Patent Document 8). Still further, in order to improve battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 9). Further, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on a superficial layer of a silicon material (see, for example, Patent Document 10).
Still further, in order to improve the cycle characteristics, a silicon oxide is used, and a graphite film is formed on a superficial layer thereof to impart electrical conductivity (see, for example, Patent Document 11). In this case, in the Patent Document 11, regarding a shift value obtained from a Raman spectrum of the graphite film, broad peaks appear at 1330 cm−1 and 1580 cm−1, and an intensity ratio thereof I1330/I1580 is 1.5<I1330/I1580<3.
Further, in order to improve high battery capacity and cycle characteristics, particles comprising a silicon micro crystallite phase dispersed in a silicon dioxide are used (see, for example, Patent Document 12). Still further, in order to improve overcharge and overdischarge characteristics, a silicon oxide in which an atomic ratio of silicon and oxygen is controlled to 1:y (0<y<2) is used (see, for example, Patent Document 13).
Further, in order to improve the high battery capacity and initial efficiency, there is a method in which an alloy-based material is contacted with a solution containing an alkali metal and a polycyclic aromatic compound, followed by soaking in a solution that desorbs an alkali metal element (see, for example, Patent Document 14).